Method for ascertaining a clutch characteristic variable by means of an electric motor

ABSTRACT

A method ascertains a characteristic variable of a clutch installed into the powertrain of a vehicle for transmitting torque between a clutch input and a clutch output. A first electric motor is connected to the clutch input to introduce a first drive torque into the clutch. The torque is ascertained when the vehicle is at a standstill in that the clutch is first opened; the first electric motor is regulated at a first rotational speed; the clutch output is regulated at a second rotational speed; a counter torque which counteracts the transmission torque is applied to the clutch output; the clutch is then closed in order to assume a slipping state in which a specific differential rotational speed between the clutch input and the clutch output is present; the first drive torque is then ascertained; and the transmission torque is determined on the basis of the first drive torque.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the U.S. National Phase of PCT Appln. No.PCT/DE2019/100907 filed Oct. 17, 2019, which claims priority to DE102018127034.8 filed Oct. 30, 2018, the entire disclosures of which areincorporated by reference herein.

TECHNICAL FIELD

The disclosure relates to a method for ascertaining a characteristicvariable of a clutch installed into a powertrain of a vehicle.

BACKGROUND

For automatically actuated clutches, knowledge of the characteristicvariables on which the actuation of the clutch depends is of crucialimportance for achieving good driving comfort and for ensuring theoperational safety of the clutch and the vehicle. The automated orpartially automated actuation of the clutch is usually carried out by aclutch actuator in which an actuating unit is displaced along a travelpath and thereby actuates the clutch. The transmitted transmissiontorque of the clutch depends on the position of the actuating unit andon the coefficient of friction of the friction lining(s) of the clutch.This relationship between the transmission torque and the travel pathcorresponds to a clutch characteristic line that is stored in a clutchcontrol of the clutch.

These parameters can change during the operation of the vehicle, forexample due to temperature fluctuations, wear, or fatigue processes inthe components. For reliable operation of the clutch, however, knowledgeof the characteristic variables is of great importance. While the travelpath can be evaluated and readjusted via a sensor system, thecoefficient of friction of the friction linings, and thus thetransmission torque, is more difficult to ascertain.

In WO 2016/008463, a method for adapting a coefficient of friction of ahybrid disconnect clutch is proposed. The hybrid disconnect clutchconnects an internal combustion engine to an electric motor. When theinternal combustion engine is running, the clutch is first opened andthen closed far enough for the clutch to be operated in a slippingmanner. The output speed on the powertrain remains constant here. Theadaptation of the coefficient of friction then takes place as a functionof the position of the actuating unit from which the clutch is operatedwith slip.

SUMMARY

It is desirable to improve a method for ascertaining a characteristicvariable of a clutch installed into a powertrain of a vehicle. Inparticular, this should be ascertained while the vehicle is at astandstill.

Accordingly, a method is proposed for ascertaining a clutchcharacteristic variable of a clutch installed into the powertrain of avehicle for transmitting a transmission torque between a clutch inputand a clutch output. A first electric motor is connected to the clutchinput in order to introduce a first drive torque into the clutch. Thetransmission torque is ascertained when the vehicle is at a standstillin that the clutch is first opened, the first electric motor isregulated at a first rotational speed, the clutch input rotates at thefirst rotational speed, the clutch output is regulated at a secondrotational speed, a counter torque which counteracts the transmissiontorque is applied to the clutch output, the clutch is then closed inorder to assume a slipping state in which a specific differentialrotational speed between the clutch input and the clutch output ispresent, said differential rotational speed being formed from the firstand second rotational speed, the first drive torque is then ascertained,and the transmission torque is determined on the basis of the firstdrive torque.

As a result, the characteristic variable can be ascertained while thevehicle is at a standstill. Furthermore, the effect on the vehicleresulting from the implementation of the method can be so small thatthis method can be performed unnoticed from the outside. The method canbe carried out more often and the characteristic variable can beascertained at shorter time intervals. The reliability of the clutch canbe increased and the operational safety and driving comfort of thevehicle can be improved.

The transmission torque can be ascertained to be equal to a differencebetween the first drive torque and a counteracting drive counter torque,for example a friction torque.

The powertrain can be a hybrid powertrain. The first electric motor canform a drive element which can deliver the first drive torque to vehiclewheels for the movement of the vehicle.

The counter torque is preferably greater than the transmission torque.

In a preferred embodiment of the invention, a second electric motor,which provides a second drive torque, is connected to the clutch output.The second electric motor can be connected to the clutch output in arotationally fixed manner.

The second electric motor may be regulated to a second rotational speed.The second rotational speed can be less than the first rotational speed.In particular, the second rotational speed can be zero. The secondelectric motor can regulate the second rotational speed to zero.Additionally or alternatively, a brake can maintain the secondrotational speed at zero. The brake can be, for example, a transmissionbrake or a wheel brake.

An internal combustion engine may be connected to the clutch input toeffect a further drive torque. The internal combustion engine can beineffective but freely rotating and have a drag torque, wherein thetransmission torque can be ascertained as the difference between thefirst drive torque and the drag torque. The first drive torque ispreferably greater than the drag torque.

The transmission torque can be smaller than the first drive torque.

The clutch may be a disconnect clutch, in particular a K0 clutch. Thedisconnect clutch can separate the internal combustion engine and anelectric motor, here in particular the second electric motor, from oneanother or connect them to one another.

The drag torque may be ascertained when the clutch is still open byascertaining the first drive torque of the first electric motor rotatingat a third rotational speed before closing the clutch and ascertainingthe drag torque as the first drive torque. The third rotational speed ofthe electric motor is kept constant or approximately constant. The thirdrotational speed can be the same as the first rotational speed.

The transmission torque and the rotational speed difference from thesecond and first rotational speed may be used to ascertain a coefficientof friction. The specific coefficient of friction can be used to adaptthe clutch characteristic line.

The characteristic variable of the clutch stored in a clutch control ofthe clutch may be adapted as a function of the characteristic variableof the clutch ascertained. For example, the ascertained coefficient offriction can be stored or renewed in the clutch control.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages and advantageous embodiments result from thedescription of the figures and the drawings.

The method is described in detail below with reference to the drawings.Specifically:

FIG. 1: shows a powertrain of a vehicle with a clutch, thecharacteristic variable of which is ascertained according to the method.

FIG. 2: shows a flowchart of the method.

FIG. 3: shows a powertrain of a vehicle with a clutch, thecharacteristic variable of which is ascertained according to the method.

DETAILED DESCRIPTION

FIG. 1 shows a powertrain 10 of a vehicle with a clutch 12, thecharacteristic variable of which is ascertained according to a method.The clutch 12 is installed into the powertrain 10 and effects atransmission of a transmission torque between a clutch input 14 and aclutch output 16. The transmission torque is the torque transmitted viathe clutch 12.

A first electric motor 18 is connected to the clutch input 14 in arotationally fixed manner for introducing a first drive torque into theclutch 12. A second electric motor 20 can effect a second drive torqueand is connected to the clutch output 16 in a rotationally fixed manner.The first electric motor 18 and the second electric motor 20 areconnected in series when the clutch 12 is closed and can deliver theirfirst and second drive torques to an output 24, for example to a vehiclewheel.

An internal combustion engine 22 is connected to the clutch input 14 ina rotationally fixed manner to effect a further drive torque. Theinternal combustion engine 22 is connected in series with the firstelectric motor 18. The clutch 12 is designed as a disconnect clutch, inparticular as a K0 clutch, and can connect the internal combustionengine 22 and the first electric motor 18 to the second electric motor20 and the output 24. The internal combustion engine 22 has a certaindrag torque, among other things due to the friction of the pistons inthe cylinders.

FIG. 2 shows a flow chart of a method 100. The characteristic variableof the clutch to be ascertained for the clutch installed into thepowertrain of the vehicle can be, for example, the transmission torque.The transmission torque is ascertained while the vehicle is at astandstill, in that the clutch is first opened 102, the first electricmotor is then regulated 104 to a specified first rotational speed 106,for example 100 rpm, and the clutch input thus rotates 108 at the firstrotational speed 106.

At the same time, before the regulation 104 of the first rotationalspeed 106 or afterwards, the clutch output is regulated 112 to a definedsecond rotational speed 110 so that the clutch output rotates 114 at thesecond rotational speed 110. Then a counter torque counteracting thetransmission torque is applied 115 to the clutch output. The clutch isthen closed 116 in order to assume a slipping state in which apredetermined rotational speed difference 118, formed from the firstrotational speed 106 and the second rotational speed 110, is presentbetween the clutch input and the clutch output. The first drive torqueis then ascertained 120 and the transmission torque is ascertained 122as a function of the first drive torque.

The internal combustion engine can be ineffective but freely rotatingand have a drag torque, wherein the transmission torque is thenascertained as equal to a difference between the first drive torque anda counteracting drive torque, here the drag torque. The first drivetorque is greater than the drag torque and the transmission torque.

As a result, the characteristic variable of the clutch can beascertained while the vehicle is at a standstill and the effect on thevehicle resulting from performing the method 100, with the firstelectric motor operated almost silently, can be so small that thismethod 100 can be performed unnoticed from the outside. The method 100can be carried out more often and the characteristic variable can beascertained at shorter time intervals. The reliability of the clutch canbe increased and the operational safety and driving comfort of thevehicle can be improved.

FIG. 3 shows a powertrain 10 of a vehicle with a clutch 12, thecharacteristic variable of which is ascertained according to a method.The first electric motor 18 is regulated to a first rotational speed106. The second electric motor 20 connected to the clutch output 16 isregulated to a second rotational speed 110, which is zero. The secondelectric motor 20 can provide a counter torque 125 counteracting thetransmission torque 124 at the clutch output 16. The counter torque 126here is greater than the transmission torque 124.

As a result of the rotational speed regulation of the first electricmotor 18, the transmission torque 124 is equal to the first drive torque126 of the first electric motor 18 minus a counteracting drive countertorque, for example a drag torque 128 caused by the internal combustionengine 22. The first drive torque 126 is preferably greater than thedrag torque 128 in order to enable a transmission torque 124.

The drag torque 128 can be ascertained with the clutch 12 still open byascertaining the first drive torque 126 of the first electric motor 18rotating at a third rotational speed, for example the first rotationalspeed 106, and ascertaining the drag torque 128 as the first drivetorque 126 before the clutch 12 is closed.

LIST OF REFERENCE SYMBOLS

-   10 Powertrain-   12 Clutch-   14 Clutch input-   16 Clutch output-   18 Electric motor-   20 Electric motor-   22 Internal combustion engine-   24 Output-   100 Method-   102 Open clutch-   104 Rotational speed control-   106 First rotational speed-   108 Rotation at first rotational speed-   110 Second rotational speed-   112 Rotational speed control-   114 Rotation at second rotational speed-   115 Provision of counter torque-   116 Close clutch-   118 Rotational speed difference-   120 Ascertain drive torque-   122 Ascertain transmission torque-   124 Transmission torque-   125 Counter torque-   126 Drive torque-   128 Drag torque

1. A method for ascertaining a clutch characteristic variable of aclutch installed into a powertrain of a vehicle for transmitting atransmission torque between a clutch input and a clutch output, a firstelectric motor being connected to the clutch input in order to introducea first drive torque into the clutch, wherein the transmission torque isascertained when the vehicle is at a standstill in that the clutch isfirst opened, the first electric motor is regulated at a firstrotational speed, the clutch input rotates at the first rotationalspeed, the clutch output is regulated at a second rotational speed, acounter torque which counteracts the transmission torque is applied tothe clutch output, the clutch is then closed in order to assume aslipping state in which a specific differential rotational speed, formedfrom the first and second rotational speeds is present between theclutch input and the clutch output; the first drive torque is thenascertained and the transmission torque is determined on the basis ofthe first drive torque.
 2. The method according to claim 1, wherein asecond electric motor is connected to the clutch output and provides asecond drive torque.
 3. The method according to claim 2, wherein thesecond electric motor is regulated to the second rotational speed. 4.The method according to claim 1, wherein the second rotational speed islower than the first rotational speed.
 5. The method according to claim1, wherein an internal combustion engine is connected to the clutchinput to effect a further drive torque.
 6. The method according to claim5, wherein the internal combustion engine is not generating power buthas a drag torque, wherein the transmission torque is ascertained as thedifference between the first drive torque and the drag torque.
 7. Themethod according to claim 6, wherein the drag torque is ascertained whenthe clutch is still open by ascertaining the first drive torque of thefirst electric motor rotating at a third rotational speed before closingthe clutch and ascertaining the drag torque as the first drive torque.8. The method according to claim 1, wherein the clutch is a disconnectclutch, in particular a K0 clutch.
 9. The method according to claim 1,wherein the transmission torque and the rotational speed difference fromthe second and first rotational speeds are used to ascertain acoefficient of friction.
 10. The method according to claim 1, wherein acharacteristic variable stored in a clutch control is adapted as afunction of the characteristic variable ascertained.
 11. The methodaccording to claim 4, wherein the second rotational speed is equal tozero.
 12. A powertrain comprising: an internal combustion engine havingan output shaft; a first electric motor having a rotor driveablyconnected to the output shaft; a second electric motor; a clutchselectively coupling the output shaft to a rotor of the second electricmotor; and a controller programmed to: regulate the first electric motorat a first rotational speed, regulate the second electric motor at asecond rotational speed, apply a counter torque with the second electricmotor, close the clutch to assume a slipping state in which a specificdifferential rotational speed, formed from the first and secondrotational speeds is present between the clutch input and the clutchoutput, and ascertain a coefficient of friction on the basis of thecounter torque and the differential rotational speed.
 13. The powertrainaccording to claim 12, wherein the second rotational speed is lower thanthe first rotational speed.
 14. The powertrain according to claim 13,wherein the second rotational speed is equal to zero.